Total hip arthroplasty (also known as a hip replacement) is a commonly performed surgical procedure which involves removing part of a patient's hip joint and replacing the hip joint with metal and/or plastic components. In a typical surgery, the surgeon will often plan the proposed surgical procedure, including estimating the location of the proposed bone cuts (osteotomy) on the templated radiographs before the surgery. The location of the bone cuts on the femoral neck will desirably partially determine the femoral prosthesis location and the patient's ultimate leg length. It is therefore important in an existing hip surgery for the surgeon to make the femoral neck bone cut in the proper location to avoid limb length inequalities.
In general, the femoral bone must be prepared in the appropriate manner with the proper position and angle to accept the intended femoral prosthesis. The femoral prosthesis should also desirably be implanted in the proper position and angle. Errors in either the preparation of the femoral bone or the implantation of the femoral prosthesis can cause leg length discrepancies, offset discrepancies, leg rotational issues, hip pain and/or hip instability issues. Typically, the femoral prosthesis should be positioned down the center of the femoral canal. If the femoral prosthesis is angled within the femoral canal such that the distal tip of the femoral prosthesis is pointing toward the lateral femoral cortex, then the femoral prosthesis is said to be in a varus position. If the femoral prosthesis is pointing toward the medial femoral cortex, then the implant is said to be in a valgus position. Ideally, the distal tip of the femoral prosthesis is pointing down the center of the femoral canal. If a femoral prosthesis is implanted in a varus or valgus position, then the implant may not rest at the appropriate level in the femoral canal, which can alter the leg length and offset. Femoral prosthesis that are implanted in a varus or valgus position may also have a higher failure rate (aseptic loosening, thigh pain, etc.) than a femoral prosthesis that is well sized and well positioned.
The native femoral anteversion is the angle formed between the femoral head and the knee joint as looking down on top of the femoral bone. Desirably, the femoral prosthesis should fit this native femoral anteversion in most situations. Unfortunately, surgeons can accidentally change the rotation of the femoral prosthesis during the preparation of the femoral bone, which can lead to bony impingement, fractures, and/or hip dislocations.
To date, surgical approaches for hip and knee replacements are often fundamentally different in terms of how they are attached to their respective bones. Knee replacements typically are attached to the exterior of the femoral and tibial bone like a cap on the end of the bone. In contrast, hip replacements are typically attached to the inside (i.e., endosteal surface) of the medullary canal. Of course, various exceptions to this general rule exist, such as hip resurfacing (where the femoral component is attached to the exterior of the femoral bone) or knee revision procedures (where a femoral post may be employed). But where the general rule applies, it aptly accounts for why femoral prosthesis can subside into the femoral canal after implantation whereas knee replacement and hip resurfacing prosthesis typically do not subside. Moreover, the fit of the femoral prosthesis inside the femoral canal is not as obvious to the surgeon with hip replacements compared to knee replacements, often because the implant is not visible. In many cases, an undersized or mal-aligned femoral prosthesis in traditional hip replacements can settle further down the femoral canal once the patient starts to walk on the implant.
Many hip replacements have a femoral prosthesis with a collar or ledge that extends outward at the junction of metaphyseal and neck portion of the femoral prosthesis. If properly positioned, this collar could rest against the femoral neck osteotomy so that the femoral prosthesis would resist subsiding down the femoral canal further than was expected. In such a design, the force transmitted across the hip joint could be partially transmitted to the femoral bone through this collar. However, because in this design the femoral prosthesis still loads the femoral bone from inside the bone, the compression of the femoral component into the femoral canal creates hoop stresses that can split or fracture the femoral bone in much the same way as a log splitter can split apart a log.
Performing a joint replacement with patient specific instruments involves obtaining a pre-operative scan of the joint and then manufacturing tools or patient specific guides that precisely fit the bone involved in the joint replacement. The patient specific instruments form a reverse mold of the surface of the bone. When the patient specific guide intimately contacts the femoral bone, the surgeon can be assured the bone cuts are being performed as planned from the pre-operative scan.